The use of an H-switch in AC frequency conversion is known. In one type of single phase cycloconverter, an H-switch connects a load to a pair of AC power lines L1 and L2 supplied by an AC source. The H-switch has one ON state connecting L1 to the left end of the load and connecting L2 to the right end of the load. The H-switch has another ON state connecting L2 to the left end of the load and connecting L1 to the right end of the load. The H-switch module has four switches, one pair of which is closed for the one ON state, and the other pair of which is closed for the other ON state.
In a typical cycloconverter, the H-switch is switched between its two ON states at a specific periodic frequency which mixes with the incoming frequency of the AC signal in heterodyning relation to yield various output frequencies, analogously to side bands. For example, mixing a 60 hertz input AC signal with a 132 hertz timing switching signal to the H-switch results in a plurality of output frequencies, including a 72 hertz output frequency, a 192 hertz output frequency, a 336 hertz output frequency and so on, Static Power Frequency Changers, Gyugyi and Pelly, John Wiley & Sons, N.Y., 1976.
The present invention departs from the classical cycloconverter approach and instead uses an irregular timing switching signal to toggle the H switch between its two ON states. This irregular timing of H-switch switching yields a chopped sinusoid output waveform of a given fundamental frequency.
In the classical cycloconverter approach, during a given half cycle of the output frequency, the output waveform may be constituted by a plurality of segments of both positive and negative polarity, for example the above noted Gyugyi reference, pages 56, 57 and 168. In contrast, in the present invention the output waveform in any half cycle is constituted by segments of common polarity.
In preferred form, the H-switch is toggled in response to a given frequency clock signal and is also toggled in response to each zero crossing of the input AC signal except when a zero crossing and a clock signal coincide in time.
The invention is particularly useful for up conversion in certain motor control applications, specifically where an increase in frequency is desired for only short periods of time compared with normal lower frequency run-time. An example is refrigeration control where the compressor must be designed for the worst case situation even though such worst case occurs perhaps only 1% of the time, for example when a freezer must cool down a whole new supply of food. During the other 99% of the time, the compressor must only maintain an already cool condition, and thus may only need perhaps half its capacity. One solution to this over-capacity is to use a smaller compressor and run it at normal speed for normal duty, and run it at a higher speed during the small percentage of time needed for higher capacity cooling, i.e. during the 1% cooldown time. This faster speed operation is not detrimental to the compressor for short periods of time.
In the present invention, the AC frequency may be increased in a simple manner for running the compressor at a faster speed. A trade off in the present frequency conversion technique is that the resultant chopped sinusoid output waveform of increased frequency is less efficient than the input AC frequency. This less efficient use of electrical power is far outweighed by the reduction in compressor capacity enabled thereby. Furthermore, during the 99% normal run-time, a smaller compressor is driven by a smaller motor at its most efficient load rating.